Summary of Work: The current investigation has been undertaken to determine, in vivo, if nitric oxide is responsible for cytokine-induced myocardial depression. The negative inotropic effect of cytokines on the heart is believed to be mediated by nitric oxide, based on in vitro data In isolated hamster cardiac papillary muscle, this negative inotropic effect of cytokines can be blocked by N-g monomethyl-L-arginine (NMA), a nitric oxide synthase inhibitor. Because the in vitro data show that nitric oxide synthase inhibitors prevented tumor necrosis factor CFNF)- induced myocardial depression of rapid onset and reversal, we studied a low dose of recombinant human TNF challenge in canines. This TNF dose produces significant, early, and short-lived myocardial depression (resolved by 24 h). Surprisingly, we found that NMA did not prevent the early (up to 6 h) deleterious effects of TNF on cardiac function. In fact, during this time period, TNF and NMA effects on all cardiac hemodynamic and metabolic parameters were additive (i.e., NMA did not block TNF effects). However, 24 h after TNF infusion, NMA did ameliorate the effects of TNF on some parameters such as acid-base derangements and decreases in mean arterial pressure and systemic vascular resistance. These data suggest that the early phase of TNF-induced cardiac and other abnormalities may not be related to nitric oxide production. However, later, some deleterious effects of TNF may be related to production of nitric oxide. Given the finding suggestive of a beneficial effect of NMA at 24 h post-TNF infusion, we are not evaluating the effect of nitric oxide inhibition in the setting of higher doses of TNF, and longer- lasting myocardial depression. Previous experiments using TNF challenges in canines suggest that two phases of cardiac injury is a reasonable hypothesis. In canines, there is an early (<8 h), dose-independent mechanism of myocardial depression and a late (>24 h), dose-dependent mechanism of myocardial depression. It is possible that inhibition of nitric oxide is not advantageous early, when myocardial depression is dose-dependent. We are now using TNF DOSES OF 45 mg/kg, and pretreating animals with doses of NMA of 40 mg/kg bolus followed by continuous infusion of 40 mg/kg/h. We believe that this investigation could provide information about the mechanism of the deleterious effects of TNF on cardiac function and could potentially provide the basis for the development of new therapeutic strategies for the treatment of the cardiovascular and hemodynamic derangements of sepsis. At present, cancer and AIDS therapy with cytokines such as interleukin-2 is limited by cardiovascular depression. NMA is presently being used with cytokine therapies for cancer patients in order to inhibit their cardiovascular toxicities, and studies are planned in AIDS patients to do the same. These studies will also help determine the advisability of this approach.